Alkylaromatic compounds, such as ethylbenzene, ethyltoluene, isopropylbenzene, and the like are very important as precursors from which vinylaromatic monomers are made. The resulting vinylaromatic monomers are used to make a variety of useful polymer materials, e.g., styrenic resins. In a typical commercial process, alkylaromatic compounds are produced by catalytic alkylation at elevated temperatures. Heretofore, two major difficulties have been encountered in catalytic alkylation vapor phase processes.
The first major difficulty is the production of unwanted by-products. For example, in the production of ethyl-benzene, significant amounts of xylene isomers are also produced. Even small amounts of xylene are undesirable because separation of the xylene isomers from the ethylbenzene is exceedingly difficult due to the fact that the boiling points of the ethylbenzene and xylene isomers are substantially the same. Another problem is the attachment of more alkyl groups to an aromatic nucleus than is desired. For example, in the production of ethyltoluene a significant amount of trimethylbenzene is also produced. Again, separation of the desired product from the unwanted by-product is extremely difficult. This phenomenon, hereinafter referred to as polyalkylation, further reduces the yield of desired product. It is therefore needful, insofar as possible to prevent the formation of xylene isomers or other undesirable by-product materials during the alkylation reaction.
The second major difficulty encountered in conventional catalytic alkylation processes is rapid loss of catalytic activity. As the catalytic reaction proceeds, the activity of the catalyst in terms of the percent of the feed which is actually converted to the desired alkylaromatic product compounds progressively decreases so that after a period of time, it becomes necessary to shut down the alkylation and either replace or regenerate the catalyst. The consequent reduction in the productivity of the equipment utilized and the expense of catalyst replacement or regeneration, substantially increase the cost of producing the desired alkylaromatic compounds. As employed herein, the term "stability" refers to the ability of the catalyst to convert feedstocks to desired products measured as a function of time during which the reactions proceed.
Recently, synthetic aluminosilicate catalysts have been recognized to be useful in alkylation processes. In particular, catalysts of the ZSM-5 series have been reported to provide advantages in alkylation procedures. Such aluminosilicate materials are subject to degeneration by coking. Further, these materials are not steam stable in the sense that they are reported to rapidly lose activity when steam or water is present during the reaction. It is generally believed that the activity of these catalyst is directly proportional to the aluminum concentration and that steam progressively dealuminates the framework, thereby irreversibly deactivating the catalyst. It has also been theorized that the water combines with the aluminum present in such compositions and adversely affects the catalytically active sites of such catalysts.
There is a continuing need for an improved alkylation process and apparatus which can overcome these problems associated with the prior art.